This invention relates to a television display system for displaying television pictures, such as transmitted by broadcast television, using a display device such as a cathode ray tube which operates by raster scanning on a field and line basis.
In a television display system the display unit is required to fill the gaps or interpolate between the samples taken by the field and line scanning. This scanning effectively samples the scene vertically (by means of a succession of lines) and temporally (i.e. in time, by means of a succession of fields). The overall response of the display unit is determined by a combination of the scanning spot profile and the persistence characteristic of the screen, and the scan rates.
Failure to interpolate adequately results in various impairments, chief of which are: large-area flicker at the field frequency, `twittering` of sharp horizontal edges at the picture frequency (assuming 2:1 interlaced scanning), static line structure of the picture, and travelling line structure of the field (line crawl). These effects are undesirable, especially if the display fills much of the visual field, as the line structure is then more easily visible and the eye perceives flicker more easily in the peripheral region of the field of view. Morever the critical flicker frequency, above which the eye does not perceive flicker, increases as the display luminance increases and, over the years, displays have increased in brightness to the point where the field frequency of conventional television, (50 Hz in the UK), is too low to prevent annoying flicker even on ordinary domestic displays.
We have appreciated that conventional cathode ray tube displays, by themselves, are inherently incapable of performing this interpolation function adequately because their spot profiles and persistence characteristics are subject to severe physical contraints which do not satisfy the conditions for ideal interpolation.